1. Field of the Invention
The present invention relates to a brushless motor, and more particularly to a device with a digital servo circuit for appropriately controlling the output torque.
2. Related Background Art
FIG. 1 is a cross-sectional view showing an example of the brushless motor. There is shown a capstan motor as an example, in which a rotary shaft 1 is supported by a metal bearing 3 and a ball bearing 4. 2 indicates a bearing housing. Plural spiral coils 6 are arranged on a stator yoke 5, facing a multi-pole magnet 7 fixed on a rotor yoke 9.
The multi-pole magnet 7 is connected to the rotary shaft 1 through a bushing 8 and the rotor yoke 9 and is therefore rendered rotatable. An FG (frequency generation) magnet 10 is formed by ultra multi-pole magnetization of a small pitch on the outer periphery of the rotor yoke 9. A magnetic resistance element 11, serving as a magnetic detector, generates FG pulses according to the rotation of the rotary shaft 1, in combination with the FG magnet 10. The pulse width becomes shorter or longer as the rotation speed increases or decreases. A Hall device 12 detects the phase of the multi-pole magnet 7, and constitutes a part of the system for switching the current supply to the coils 6. A numeral 19 indicates the brushless motor to be controlled.
FIGS. 2A, 2B and 2C are charts showing the variation in torque generated by a brushless motor. In case of a three-phase brushless motor, the magnetic flux distribution in the coil generally assumes a sinusoidal form as shown in FIG. 2A. For magnetic poles of a number n, the coils of different phases are mutually displaced by (2/3.360/n).degree. on the stator yoke 5, thereby generating magnetic flux changes of three phases.
The above-mentioned fact and the current supply to the coils 6 of different phases at the timings shown in FIG. 2B depending on the outputs of Hall devices give rise to a torque as shown in FIG. 2C, involving a torque ripple. Consequently the rotary shaft 1 shows uneven rotation.
The unevenness in rotation speed .DELTA.N and the torque ripple T are mutually related as follows: ##EQU1## wherein J is the inertia of the rotating parts, and N is the rotation speed. The uneven rotation caused by such torque ripple is particularly marked in a capstan motor where the rotation speed is very low, so that magnetic recording and/or reproducing apparatus employing such brushless motor have resulted in wow or flutter in audio signal or jitter in the video signal. An increase in the inertia J, which has been employed as the measure for resolving this drawback, has been a serious obstacle in reducing the weight of the motor or of equipment utilizing such motor.